Process for the annealing of drawn carbon steel rolls and coils of carbon steel sheet

ABSTRACT

Improved process for the annealing of drawn carbon steel rolls and coils of rolled carbon steel sheet. This process is characterized in that in the stage of roll heating the heating is stopped before reaching the temperature at which the reaction between carbon and H 2 O in the vapour phase starts. The core of the steel roll is then allowed to heat up progressively until its temperature reaches that of the outer part of the roll. With progressive removal of moisture from the core of the steel roll, heating is applied progressively until a minimum moisture level guaranteeing a sufficiently low partial pressure to avoid the reaction between the carbon in the steel and water vapour is reached. Heating is then continued up to the desired annealing temperature.

This application is a 371 of PCT/EP98/07327 filed Nov. 11, 1998.

BACKGROUND OF THE INVENTION

(i) Field of the Invention

This invention relates to an improved process for the annealing of drawncarbon steel rolls and coils of carbon steel sheet.

(ii) Description of Related Art

One of the sectors of the steel processing industry is that of drawing.

Drawing steel comprises passing the steel through a die, which has aspecific cross-section which may be circular, square, hexagonal, etc.,in order to obtain a piece of great length and constant cross-sectionwhich is identical to the cross-section of the die. This piece, which isobtained by cold forming, is obtained by applying a specific continuouspulling force to the end.

A lubricant, which is normally sodium stearate or calcium stearate, isapplied to the material in order to ease the passage of the steel beingshaped through the die, immediately before it passes through the die.

This lubricant adheres firmly to the entire surface of the part and itsentire length.

As the pieces obtained by drawing are very long, on leaving the drawingmachine they are made into rolls of a specific weight, which assistssubsequent handling.

Pieces obtained by cold drawing undergo changes in their crystallinestructure and mechanical properties as a result of this process andcannot always be used in that state in industry.

Most frequently these pieces, as rolls, are subjected to annealing heattreatment, recrystallization heating, to restore the structure of drawnpieces and adjust their mechanical properties for subsequent processesin the industry.

When these pieces have been hot drawn—wire drawing—they are alsosubjected to globulizing heat treatment.

The annealing treatment consists of progressively heating the drawnsteel rolls from ambient temperature to a specific temperature above700° C., and once the mass of steel in the rolls reaches the specifiedtemperature they are allowed to cool slowly to ambient temperature.

If the necessary precautions are not taken during the process ofannealing the drawn steel rolls, the steel can lose some of the carbonwhich it contains, and if this occurs and the loss of carbon is greaterthan that accepted in the specifications, the quality of the steelsuffers and it cannot be used for the task for which it was intended.

This loss of carbon can only be caused by:

reaction between the carbon in the steel and oxygen,

reaction between the carbon in the steel and carbon dioxide,

reaction between the carbon in the steel and water, in the vapour state.

Of the three reactions mentioned, the one having the most marked effectsand the one which is the most difficult to avoid is the last one shown,that is the reaction between the carbon in the steel and water vapour.

Necessary precautions are taken in the industry to avoid these risks ofcarbon loss, the solution most widely used being the use of inertatmospheres, in which the component is nitrogen, or slightly reactiveatmospheres in which the basic component is nitrogen with very smallconcentrations of a hydrocarbon, normally natural gas or propane.

In this way, and with normal annealing cycles—continuous heating up tothe specified annealing temperature—reactions between the carbon in thesteel and oxygen and carbon dioxide are successfully prevented, but notthe reaction between the carbon in the steel and water vapour, so thereis a loss of carbon from the steel after treatment, and this despite arelatively high consumption of atmosphere in relation to the quantity ofsteel annealed.

It is relatively easy to eliminate the species which oxidize the carbonin the steel, oxygen (O₂) and carbon dioxide (CO₂), and this can be donewithout difficulty before the outer coils of the steel rolls reach thetemperature of 680° C., which is the temperature at which the reactionbetween the carbon in the steel and the previously mentioned specieswhich oxidize it, O₂, CO₂ and H₂O, begins.

This is not the case however for water vapour (H₂O), which is alsoanother oxidizing agent for the carbon in the steel, and whichfurthermore has more marked oxidizing effects.

In the normal cycles for annealing drawn steel rolls, when heating iscontinuous up to the annealing temperature—always above 680° C.—thefollowing take place:

When the outer turns of the drawn steel rolls reach a temperature of680° C. the centre or core of the rolls is still at a lower temperature.This temperature gradient can be 25 to 50° C. depending upon the size ofthe rolls.

This temperature gradient—with colder cores in the rolls—is sufficientto ensure that not all the moisture in the centres or cores of the steelrolls has been eliminated.

This moisture leaves the core of the steel rolls, and coming intocontact with the outer turns of the rolls at a temperature of 680° C. ormore reacts with the carbon and gives rise to a loss of carbon ordecarburization of the material.

SUMMARY AND OBJECTS OF THE INVENTION

The purpose of this invention is to improve the process described above,for which purpose it is proposed that the following four operations beperformed:

modification of the annealing cycle during the heating stage,

continuous analysis of the quantity of H₂O present in the atmosphere(within the furnace and in direct contact with the drawn steel rolls),

holding for a sufficient time for the removal of water to reach a levelsuch that the partial pressure of the said water is sufficiently low forthere to be no risk that this water will react with the carbon in thesteel, and

strongly reducing the flow of atmosphere when the analysed H₂O levelpresents no risk of reaction with the carbon in the steel.

By proceeding in this way drawn steel rolls can be annealed without anyloss of carbon and with a smaller total consumption of atmosphere thanin the normal annealing processes known hitherto.

Thus one object of this invention is a process for annealing drawncarbon steel rolls in which the steel being shaped, to which a lubricanthas previously been applied, is caused to pass through a die ofspecified cross-section, producing a roll by cold forming which issubsequently subjected to annealing heat treatment by progressivelyheating the drawn roll up to a specified temperature, after which theentire roll is allowed to cool slowly to ambient temperature,characterized in that in the heating stage heating of the steel roll isstopped before the temperature at which the reaction between the carbonin the steel and H₂O in the vapour phase begins, avoiding the saidreaction between the carbon in the steel and the water vapour, afterwhich the temperature of the core of the steel roll is allowed to becomethe same as that in the outer part of the roll, with the progressiveremoval of moisture from the core of the steel roll, until a minimummoisture level is reached which guarantees a sufficiently low partialpressure to avoid the reaction between the carbon in the steel and thewater vapour, after which heating is continued to the desired annealingtemperature, thus obtaining rolls of steel which are not decarburized.

According to the invention, heating of the drawn roll is stopped at atemperature of between 620° C. and 670° C.

Likewise, according to this invention, the annealing temperature is 680°C. or higher.

In accordance with the invention, during the annealing process themoisture content in the form of water vapour present in the atmospherewithin the annealing furnace in direct contact with the rolls of drawnsteel is continually analysed, as a result of which if the H₂O level issufficiently low heating is continued to the specified annealingtemperature and the incoming flow of atmosphere into the annealingfurnace is reduced, and if the level of H₂O is higher than the valuewhich is considered to be without risk of reaction with the carbon inthe steel, the incoming flow of atmosphere into the annealing furnace isincreased.

The process described above can likewise be applied to the annealing ofcarbon steel sheet coils when it is desired to avoid the loss of carbonfrom the sheet, and likewise this process must also be regarded as anobject of this invention.

The process according to this invention will be described in greaterdetail below with the help of the accompanying drawings. It shouldhowever be understood that these drawings show a particularly preferredembodiment of the process according to the invention which should not beregarded as limiting it in any way.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A: a detail in transverse cross-section of the furnace, the innerspace and the roll of steel during the annealing process in an annealingfurnace according to the known state of the art,

FIG. 1B: a graph of temperature as a function of time showing theannealing cycle in a process according to the known state of the art,

FIG. 2A: a detail of the furnace, internal space and the steel roll intransverse cross-section during the process of annealing in an annealingfurnace according to the invention, and

FIG. 2B: a graph of temperature as a function of time showing theannealing cycle in a process according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

With reference to FIG. 1A in the drawings, this shows a transversecross-sectional view of an annealing furnace in which a drawn carbonsteel roll is being annealed, an annealing which in reality comprises aheat treatment process with a view to softening the steel withoutaltering its surface chemical composition. In this figure (TR) indicatesthe tube radiating energy and (CI) constitutes the internal space of theannealing furnace. As may be seen, in the heart of the steel roll theturns (RAN) are at a temperature of 650° C. and furthermore a greatquantity of H₂O is present in an atmosphere (AT) of N₂, but despite thisthe turns (RAN) do not become decarburized because their temperature isbelow 680° C. However, at the same time the outer turns (RAE) of thesteel roll are at a temperature of 700° C. and, as a large quantity ofH₂O leaving the core of the steel roll is present in the N₂ atmosphere(AT), this H₂O reacts with the carbon in the outer turns of the steelroll because these turns are at a higher temperature than thetemperature at which the reaction starts (680° C.) and the steel becomesdecarburized.

FIG. 1B shows a graph of temperature as a function of time in theannealing cycle for drawn carbon steel rolls, in which it will be seenthat in this case heating of the steel rolls takes place withoutinterruption from ambient temperature up to the annealing temperature,which corroborates the fact that at least some partial decarburizationof the rolls can take place in these rolls.

With reference to FIG. 2A, this shows the same cross-section as in FIG.1A, but in this case all the turns in the roll of steel are at the sametemperature of 650° C., both the turns (RAN) in the heart of the steelroll and the turns (RAE) on the outside of the steel roll, and inaddition to this there is a smaller quantity of H₂O present in theatmosphere (AT) of the furnace than in the case of the known annealingprocess in the prior art. This smaller quantity of water in theatmosphere is due to the fact that heating of the steel rolls wasstopped at a temperature of 650° C. and held at this temperature for asufficient time for the H₂O to leave the core of the steel rolls and forthe H₂O to leave the atmosphere of the furnace for the exterior at thesame time. This is what is shown in the graph in FIG. 2B, in which itcan be seen that heating is stopped during the heating stage, forexample at 650° C., before reaching the temperature at which thereaction between the carbon in the steel and H₂O begins. By working inthis way the outer turns in the rolls will not exceed this temperature,with the result that there will be no reaction between the carbon in thesteel and water.

On the other hand, in the core of the rolls, which is colder, heatingwill take place progressively until the temperature becomes equal tothat in the outer turns. At the same time moisture progressively leavesthe interior, and is evacuated from the annealing furnace by dilutionand purging (H₂O-free atmosphere is continually entering the furnace andthe same quantity of atmosphere+H₂O is leaving it).

This is only achieved over a period of time, and this will be the timerequired to achieve the same temperature (somewhere between 620 and 670°C.) throughout the mass of the rolls, and the time required to removeall the moisture from the interior of the furnace or the moisturecontent necessary—as shown by analysis—to achieve a minimum level whichensures a partial pressure that is sufficiently low not to give rise tothe reaction with the carbon in the steel when heating of the entiremass is subsequently continued and a temperature of 680° C. or higher isreached.

Results obtained with a drawn carbon steel annealed in accordance with aknown process according to the prior art and those obtained with theprocess according to this invention indicate that a decarburized zonemay be present in the drawn carbon steel roll after the annealingprocess of the prior art, a zone which is not observed in the roll ofsteel which was annealed in accordance with the process according to theinvention.

The process of annealing to which this invention relates makes use ofequipment for continuously analysing moisture content and when thismeasures a level of H₂O which is sufficiently low not to bring about theloss of carbon from the steel it emits a signal which makes it possibleto:

continue heating up to the specified annealing temperature,

reduce the incoming flow of atmosphere to the furnace.

If during the rest of the annealing cycle up to the specifiedtemperature, during holding at this annealing temperature and duringcooling down to 680° C., the analyser detects and measures an H₂O valuewhich is greater than the value considered to be without risk ofreaction with the carbon in the steel, it sends a signal to immediatelyincrease the flow of atmosphere into the furnace in order to immediatelyremove the anomalous increase in H₂O by dilution and purging, and thisis maintained until suitable conditions of zero risk of decarburizationof the steel and low atmosphere consumption are re-established.

The annealing process described above is likewise applicable to theannealing of carbon sheet steel coils when it is desired to avoid theloss of carbon from the sheet.

It should be understood that what has gone before is a merelyillustrative description of the subject matter of this invention andthat a number of modifications may be made therein by those skilled inthe art which should be regarded as falling within the scope of theinvention which is only limited by the appended claims.

What is claimed is:
 1. A process for annealing drawn carbon steel rollscomprising the steps of: (i) passing steel being shaped, to which alubricant has previously been applied, through a die of specifiedcross-section, producing a steel roll including a core and an outer partby cold forming; (ii) subsequently subjecting said roll to an annealingheat treatment by progressively heating the drawn roll up to a specifiedtemperature, (iii) stopping the heat treatment before reaching atemperature at which a reaction between the carbon in the steel and H₂Oin a vapor phase begins, thereby avoiding the reaction between thecarbon in the steel and the water vapor, (iv) allowing the core of thesteel roll to heat progressively until its temperature becomes the sameas that in the outer part of the roll, while progressively removingmoisture from the core of the steel roll, until reaching a moisturelevel which generates a sufficiently low partial pressure to avoidreaction between the carbon in the steel and the water vapor; (v)continuing heating of the roll up to a desired annealing temperature,thus obtaining rolls which are not decarburized; and (vi) allowing theentire roll to cool slowly to ambient temperature.
 2. The processaccording to claim 1, comprising stopping the heat treatment of thedrawn roll at a temperature between 620° C. and 670° C.
 3. The processaccording to claim 1, wherein the desired annealing temperature is 680°C. or higher.
 4. The process according to claim 1, further comprisingthe step of (i) continually analyzing the moisture content in the formof water vapor present in an atmosphere in direct contact with the rollsof drawn steel during said annealing heat treatment, (ii) if the H₂Olevel is sufficiently low, continuing the heating up to the specifiedannealing temperature and reducing an incoming flow of atmosphere intothe annealing surface, and (iii) if the level of H₂O is higher than avalue which is considered to be without risk of reaction with the carbonin the steel, increasing the incoming flow of atmosphere during theannealing heat treatment.
 5. A process for annealing carbon steel sheetcoils comprising the steps of: (i) passing steel which has to be shaped,to which a lubricant has previously been applied, through a rolling millin order to obtain a specific cross-section, yielding a coil of steelsheet, including a core and an outer part, through cold forming; (ii)subjecting said coil of steel sheet to an annealing heat treatment byprogressively heating the coil of rolled sheet steel up to a specifictemperature, (iii) stopping the heating of the sheet steel coil beforereaching a temperature at which a reaction between carbon in the steeland H₂O in the vapor phase begins, avoiding the reaction between thecarbon in the steel and the water vapor, (iv) allowing the core of thesteel coil to heat up progressively until its temperature becomes thesame as that in the outer part of the steel coil, with the progressiveremoval of moisture from the core of the steel coil, until a moisturelevel is reached having a sufficiently low partial pressure to avoid areaction between the carbon in the steel and the water vapor, (v)continuing the heating up to the desired annealing temperature, (vi)cooling the entire coil of steel sheet, thus obtaining coils of steelwhich are not decarburized.
 6. The process according to claim 1, whereinthe only annealing gas introduced during the annealing heat treatment isnitrogen.
 7. The process according to claim 5, wherein the onlyannealing gas introduced during the annealing heat treatment isnitrogen.
 8. The process according to claim 1, further comprising thestep of analyzing the moisture content in the form of water vaporpresent in an atmosphere in direct contact with the rolls of drawn steelduring said annealing heat treatment.
 9. The process according to claim5, further comprising the step of analyzing the moisture content in theform of water vapor present in an atmosphere in direct contact with thecarbon steel sheet coils during said annealing heat treatment.
 10. Theprocess according to claim 5, further comprising the step of (i)continually analyzing the moisture content in the form of water vaporpresent in an atmosphere in direct contact with the carbon steel sheetcoil during said annealing heat treatment, (ii) if the H₂O level issufficiently low, continuing the heating up to the specified annealingtemperature and reducing an incoming flow of atmosphere into theannealing surface, and (iii) if the level of H₂O is higher than a valuewhich is considered to be without risk of reaction with the carbon inthe steel, increasing the incoming flow of atmosphere during theannealing heat treatment.